Optimize StringSequence.startsWith

See gh-21259

spring-boot-project/spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/jar/StringSequence.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -98,15 +98,19 @@ final class StringSequence implements CharSequence {
 		return this.source.indexOf(str, this.start + fromIndex) - this.start;
 	}
 
-	boolean startsWith(CharSequence prefix) {
+	boolean startsWith(String prefix) {
 		return startsWith(prefix, 0);
 	}
 
-	boolean startsWith(CharSequence prefix, int offset) {
+	boolean startsWith(String prefix, int offset) {
 		int prefixLength = prefix.length();
-		if (length() - prefixLength - offset < 0) {
+		int length = length();
+		if (length - prefixLength - offset < 0) {
 			return false;
 		}
+		if (length == this.source.length()) {
+			return this.source.startsWith(prefix, offset);
+		}
 		int prefixOffset = 0;
 		int sourceOffset = offset;
 		while (prefixLength-- != 0) {